let phrases = ["hello world", "the definitive guide"];
console.log(phrases.flatMap((v) => v.split(" "))); // [ 'hello', 'world', 'the', 'definitive', 'guide' ]
console.log([-3, -2, -1, 1, 2].flatMap((v) => (v < 0 ? [] : [Math.sqrt(v)]))); // [ 1, 1.414 ]
